It is generally believed that fever is caused not by exogenous, but by endogenous pyrogens, members of an array of peptide mediators called cytokines. These are elaborated and released into the circulation largely by systemic mononuclear phagocytes activated by the exogenous agents, and transported to the preoptic anterior hypothalamic area (POA) of the brain, where they act. Prostaglandin E2 (PGE2) is thought to be a fever mediator in the POA, induced by these cytokines. Although the intrapreoptic level of PGE2 rises rapidly following the intravenous (iv) administration of exogenous (e.g., lipopolysaccharide, LPS) or endogenous pyrogens, recent data indicate that these substances cannot account directly for the rapid production of PGE2 because it is initiated well before the synthase that they stimulate (cyclooxygenase-2, COX-2) becomes active. Moreover, cytokines also are not detectable in blood following i.v. LPS until after the febrile response is already underway, and there is no evidence that they can readily penetrate the brain. Hence, other, more quickly evoked mediators may be presumed to be involved in initiating the febrile response. Based on well-recognized mechanisms of LPS action, we have hypothesized that LPS virtually immediately activates the complement (C) cascade, generating the anaphylatoxins C3a and C5a, which function to activate macrophages and rapidly induce a mediator that has the capacity to stimulate peripheral sensory nerves that then transmit this information to the POA. We have data supporting the involvement of systemic C in the febrile response of guinea pigs to LPS, but more critical when LPS is injected i.p. than when it is administered iv. We hypothesize from preliminary data that this differential dependence on C may be related to distinct functional and biochemical characteristics of peritoneal and hepatic macrophages. Finally, recent preliminary data have suggested that the released mediator that acts on neural afferents may not be PGE2 and may also not be derived from macrophages, as had been supposed. The purpose of this proposed research, therefore, is to address the issues raised by our findings. The results will contribute to understanding the mechanisms of peripheral pyrogenic signaling, particularly regarding the onset of fever, as well as the processes that underlie the multivariate host defense reactions to infectious stimuli in general.